The present invention relates to an ignition apparatus for an internal combustion engine, and more particularly, it relates to an ignition apparatus which is capable of preventing malfunctions due to noise induced by a high voltage generated upon discharge of a spark plug. The invention also relates to an ignition apparatus which is able to discriminate intentional or controlled misfiring due, for example, to intentional cut-off of the fuel supply from other misfiring due to malfunctions.
FIG. 5 shows a typical example of a known ignition apparatus for an internal combustion engine. In this figure, the apparatus illustrated includes a controller 1 for controlling the fuel injection and the ignition of an internal combustion engine in synchronism with the rotation thereof, a power transistor 2, an ignition coil 3, a reverse-current checking diode 4, and a spark plug 5. The ignition coil 3 has a primary winding connected to ground through a collector-emitter connection of the power transistor 2, and a secondary winding connected to one electrode of the spark plug 5 through the reverse-current checking diode 4. The spark plug 5 has the other electrode connected to a negative electrode of a DC power source 8 through an ion current sensing diode 6 and a resistor 7. A serial connection comprising a capacitor 9 and a resistor 10 is connected in parallel with a serial connection comprising the resistor 7 and the DC power source 8. A comparator 11 has a pair of first and second input terminals, the first input terminal being connected to a junction between the capacitor 9 and the resistor 10, the second input terminal being connected to a reference voltage source. When a voltage D imposed on the first input terminal, as shown at (D) in FIG. 6, exceeds the reference voltage at the second input terminal, the comparator 11 generates an output signal E, as shown at (E) in FIG. 6, which is input as a reset signal to a pair of first and second counters 12, 13 which together constitute a binary counter. In this regard, the elements 6 thorough 11 together constitute an ion current detector for detecting an ion current generated between the electrodes of the spark plug 5 upon combustion of an air/fuel mixture in the cylinder 15. The first counter 12 is alternately turned on and off or turned into a high level and a low level by a clock pulse supplied thereto from a signal generator 19 through a comparator 20, which will be described in detail later, and it is reset by a reset signal E from the comparator 11, so that it generates an output signal, as shown at (F) in FIG. 6. The second counter 13 generates a high output when a clock pulse A is input to the first counter 12 during the time the first counter 12 is at a high level, and it is reset by a reset signal E from the comparator 11.
The controller 1 supplies a fuel injection control signal to a fuel injector 14 which injects, based thereon, an appropriate amount of fuel into an intake pipe IP of the engine. The engine includes a cylinder 15 in which a piston 16 is received for reciprocating movement. The piston 16 is connected with a crankshaft 18 through a piston rod 17.
A signal generator 19 generates a control signal in synchronism with the rotation of the crankshaft 18. The control signal contains a series of pulses occurring at predetermined intervals. The control signal from the signal generator 19 is fed to the controller 1 as well as the first counter 12 through the comparator 20 as a clock signal.
The operation of the above-mentioned known ignition apparatus will now be described in detail with reference to a timing chart FIG. 6 which shows the waveforms of signals at various portions of the ignition apparatus.
Under the normal operating condition of the engine in which normal combustion takes place in the cylinder 15 without misfiring, in synchronism with an output or clock pulse A from the signal generator 19, which is shown at (A) in FIG. 6, the controller 1 generates a fuel injection control signal B, as shown at (B) in FIG. 6, which is fed to the injector 14. At the same time, the controller 1 turns the power transistor 2 off so that a positive voltage is developed across the primary winding of the ignition coil 3, as shown at (C.sub.1) in FIG. 6, and a negative voltage is developed across the secondary winding of the ignition coil 3, as shown in at (C.sub.2) in FIG. 6, thereby causing the spark plug 5 to generate a spark. Upon sparking of the spark plug 5, an air/fuel mixture in the cylinder 15 is fired. As a result, between the electrodes of the spark plug 5 there is generated an ion current I which is supplied to the first input terminal of the comparator 11 through the diode 6 and the capacitor 9. The waveform of the ion current I thus supplied to the comparator 11 contains a noise component N, as illustrated at (D) in FIG. 6, which results from a high voltage induced across the secondary winding of the ignition coil 3 when the power transistor 2 is turned off. When the comparator 11 receives the ion current I containing the noise component N at the first input terminal thereof, it generates an output signal in the form of a reset signal E, as shown at (E) in FIG. 6. In other words, within one period of the clock signal A from the signal generator 19 (i.e., a period between successive clock pulses), there is generated two types of reset signals, one being due to noise and the other due to the ion current. As a consequence, the first counter 12, which is alternatively turned on and off by a clock signal pulse and is reset by a reset signal pulse, is always reset by a reset signal due to noise, so that it generates an output signal which rises at the rising edge of a clock pulse A and falls at the rising edge of a noise-induced reset pulse, as shown at (F) in FIG. 6. Accordingly, the second counter 13 generates no output or a low level output at all times, as shown at (G) in FIG. 6.
In this manner, the first and second counters 12, 13 of the known ignition apparatus operate irrespective of the presence and absence of an ion current, so when misfiring takes place at a time between time t2 corresponding to the rising edge of a clock pulse and time t3 corresponding to the rising edge of the following clock pulse, it is impossible to detect this misfiring.
In addition, if the controller 1 intentionally cuts off the fuel supply to the cylinder 15 for saving fuel during rapid decelerations for example, the second counter 13 generates a high level output indicative of misfiring in the cylinder 15. That is, as illustrated in FIG. 7, if the fuel supply to the cylinder 15 is to be cut off at a time between t.sub.3 and t.sub.4 for example, the controller 1 stops generation of a fuel injection control signal, as shown at (B) in FIG. 7, so there is no ion current generated, as shown at (D) in FIG. 7, and hence the comparator 11 generates no reset signal, as shown at (E) in FIG. 7. As a result, as illustrated at (F) in FIG. 7, the output of the first counter 12 rises at time t.sub.3, at which a clock pulse A is input thereto from the comparator 20, and falls at time t.sub.4, at which the following clock pulse A is input, so that the second counter 13 generates a high level output at time t.sub.4 and is then reset by a reset pulse E from the comparator 11 at time t.sub.6, as shown at (G) in FIG. 7. That is, the second counter 13 generates a misfiring detection signal during fuel supply cut-off periods, which is undesirable.